Radioactive waste is disposed, for instance, by radioactive waste geological disposal, in which the waste is melted into a vitreous material that is cast into iron or steel containers, the containers being then buried underground. In geological disposal of radioactive waste there are built artificial barriers (water-impervious layers) of a clayey material, with a view to reliably isolating the radioactive waste.
Known methods for building such artificial barriers include heaping of bentonite blocks, and on-site tamping. Methods for heaping up bentonite blocks involve transporting bentonite blocks manufactured in a factory or the like to a site, and then fixing the blocks by suction gripping or crane lifting. In an on-site tamping method, compaction is carried out, for instance, using a vibrating roller, a pneumatic striking hammer as an improved concrete chipper, or using a weight-drop automatic tamping machine. Methods for disposing such radioactive waste include, for instance, the waste disposal facility disclosed in Japanese Patent No. 3054728 (Patent document 1), and the backfill method and block manufacturing method used therein, disclosed in Japanese Unexamined Patent Application Laid-open No. 2000-193796 (Patent document 2).
As the clayey material there is ordinarily used a bentonite-based material having bentonite as a main constituent and comprising bentonite or a bentonite mixed soil in which bentonite contains sand or the like. Such a bentonite-based material is ordinarily used in the form of a raw material having a main constituent of bentonite powder to which there is added water to a predetermined water content.
A bentonite block heaping method, however, requires a substantial plant equipped with large jacks and the like. A method involving tamping on site is problematic in that it requires using major equipment items such as large vibration rollers and/or compacting machinery. These methods have drawbacks also in that they allow building an artificial barrier at narrow sites only with difficulty.
Spray methods using a bentonite-based material have been studied with a view to tackling the above problems. In such spray methods, a bentonite-based material is sprayed onto the inner surface of a tunnel or the like, to build thereby an artificial barrier (water-impervious layer). Conventional spray methods, directly mainly at enhancing imperviousness of a slope, are disclosed in, for instance, Japanese Unexamined Patent Application Laid-open No. 2001-81761 (Patent document 3) and Japanese Patent No. 3494397 (Patent document 4). Such spray methods do not require using large equipment such as jacks, vibrating rollers or the like, and hence allow building artificial barriers easily. They allow, moreover, building artificial barriers easily also in narrow sites.
Patent document 1: Japanese Patent No. 3054728    Patent document 2: Japanese Unexamined Patent Application Laid-open No. 2000-193796    Patent document 3: Japanese Unexamined Patent Application Laid-open No. 2001-81761    Patent document 4: Japanese Patent No. 3494397